This is a clinical study of DMP 266, an investigational drug being studied for the treatment of HIV infection in children. This study will try to find out if DMP 266 is safe and effective for children and determine the proper dosage in children. This drug will be taken in combination with the FDA approved drug Nelfinavir(NFV). The study will also determine the appropriate dose of NFV in children when given in combination with DMP 266.These medicines will be taken in addition to the usual anti-HIV medicine the patient is already taking like AZT, d4T, ddI or 3TC. The dose of the medicines may change depending on the results of these pharmacokinetic studies.